A mixer circuit is generally used for modulation and demodulation of signals in a communication system. This mixer circuit mixes at least two input signals to output a signal having a frequency corresponding to the difference between mixed two signals or the sum of the two signals.
FIG. 1 is a circuit diagram of a Gilbert cell mixer circuit that is a conventional typical mixer circuit. Referring to FIG. 1, the conventional mixer circuit includes an amplification unit 110 and a mixing unit 130. The amplification unit 110 is composed of a first NPN transistor BN11 and a degeneration impedance L11, and amplifies a signal applied to an input Vin to output it to the mixing unit 130. The mixing unit 130 includes second and third NPN transistors BN12 and BN13, first and second load resistors R11 and R12. The mixing unit 130 mixes the signal outputted from the amplification unit 110 with local oscillation signals LO+ and LO−, to output a baseband signal.
The operation of the mixer circuit shown in FIG. 1 and problems in the circuit will be described hereinafter.
The amplification unit 110 of the mixer circuit has a common emitter structure and amplifies a signal Vin applied to the base of the first NPN transistor BN11 to output it to the mixing unit 130. The mixing unit 130 of the mixer circuit has a base common structure and mixes the output signal of the amplification unit 110 with the local oscillation signals LO+ and LO− to output an intermediate frequency or baseband signal.
That is, the mixer circuit shown in FIG. 1 is constructed in such a manner that the amplification unit 110 having a common emitter structure and the mixing unit 130 having a common base structure are connected to each other and the local oscillation signals LO+ and LO− are respectively applied to the bases of the second and third NPN transistors BN12 and BN13 to be mixed with an RF signal applied to the base of the first NPN transistor BN11.
In the conventional mixer circuit shown in FIG. 1, however, it is difficult to utilize the mixer circuit in an optimal manner in terms of linearity and noise figure because the amplification unit 110 is directly connected to the mixing unit 130. Specifically, current flowing through the first NPN transistor BN11 of the amplification unit 110 has the same value as the sum of currents flowing through the second and third NPN transistors BN12 and BN13. Accordingly, when the quantity of currents flowing through the second and third NPN transistors BN12 and BN13 of the mixing unit 130 is increased in order to improve linearity of the mixer circuit, the current flowing through the first NPN transistor BN11 is also increased. This raises gain of the amplification unit 110. An increase in the gain of the amplification unit 110 deteriorates linearity of the mixer circuit. Furthermore, in the case where the current flowing through the first NPN transistor BN11 is increased while voltage Vce across the collector and emitter of the first NPN transistor BN11 is constant, linearity of the amplification unit is decreased. Moreover, it is difficult to optimize linearity and noise figure of the mixer circuit because gains of the amplification unit 110 and mixing unit 130 cannot be independently controlled.
That is, in the case where the mixer circuit is constructed in a manner that the amplification unit 110 and the mixing unit 130 are directly connected to each other although conditions for optimizing linearity and noise figure of the amplification 110 are different from them of the mixing unit 130, linearity and noise figure of the mixer circuit are deteriorated.
U.S. Pat. No. 5,532,637 proposes a mixer circuit for improving linearity and noise figure thereof. FIG. 2 shows a circuit diagram of the mixer circuit disclosed in U.S. Pat. No. 5,532,637.
Referring to FIG. 2, the mixer circuit includes three pairs of transistors 11, 12, 15, 16, 17 and 18, first and second current sources 10 and 22. The first pair of transistors 11 and 12 controls current flowing between their collectors and emitters according to a first input signal applied differentially to first input terminals I/P1+ and I/P1−. The two second pairs of transistors 15, 16, 17 and 18 control the quantity of currents flowing through them according to a second input signal applied to second input terminals I/P+ and I/P− and the current of a corresponding transistor of the first pair of transistors. The second current source 10 provides for the total current passed by the first pair of transistors 11 and 12 to be greater than the total current passed by the two second transistors 15, 16, 17 and 18.
The prior art mixer circuit shown in FIG. 2 has the current source 22 at the input terminal of the mixer to improve linearity of the mixer circuit. However, it is known that currents, supplied to the collectors of the first pair of transistors 11 and 12 after the voltage Vce across the collectors and emitters of the first pair of transistors has been determined, do not improve linearity but deteriorate noise figure and current consumption (Guofu niu, et al., “RF Linearity Characteristics of SiGe HBTs”, IEEE, Transaction of Microwave theory and Techniques, vol. 49, No. 9, September 2001).
Accordingly, in the case where the voltage between the collector and emitter is uniform, as shown in FIG. 2, there are limitations in improving linearity by providing more current to the amplification unit. Furthermore, there is also a limitation in increasing the current and the voltage across the collector and emitter, simultaneously, because of Vcc. Moreover, although larger quantity of currents flowing through the two second pairs of transistors 15, 16, 17 and 18 included in the mixing unit can obtain high linearity, linearity of the mixer circuit disclosed in U.S. Pat. No. 5,532,637 is deteriorated since the quantity of current flowing through the mixing unit is smaller than the quantity of current flowing through the amplification unit.